The present invention relates to connectivity between land-based cellular communications systems and user equipment located in airborne craft.
The smartphone has allowed people to be connected anywhere and anytime. This has led consumers to have increasing expectations of being able to be online and experience at least moderate data rates in locations that have traditionally not provided good connectivity, such as in-flight aboard an aircraft. Having shown little progress so far, the smartphone changed the need for this greater connectivity starting in the middle of the first decade of the twenty-first century. Now it is a steadily growing branch of communications, still early in its development and far from the performance seen in cellular or WiFi networks, where streaming video or other high bandwidth services totaling several hundreds of Mbps may be supplied for each eNodeB (“eNB”) or Access Point (AP). Due to limited alternative activities, a scenario with several hundreds of parallel video streams is not unlikely in an airborne device acting as a relay or access point to the user equipments (UEs) in the aircraft. Today two main technologies exist for in-flight communications, that is, communications to and from the ground to a commercial airplane. One is a ground based cellular network, the other, which comes in several variations, is satellite based.
New antenna techniques allow for higher channel capacity, for example by also utilizing the polarization plane in the transmissions such that vertical, horizontal and left or right circular polarization may be differentiated from one another. With new antenna array or planar array techniques, new possibilities for beamforming arise. By using arrays, it is possible to focus the transmitted energy in a beam in a specific direction in space. The antenna gain in the relevant direction compared to an isotropic antenna is approximately 10*log(N), where N is the number of elements in the array, ignoring antenna imperfections. A 64 element array would thus in theory have an antenna gain of 18 dBi. Combining this kind of antenna gain with an eNB output power of 46 dBm, the signal-to-noise ratio (SNR) at a range of 100 km would still be 54 dB using the free space path loss model! The line of sight (LoS) case is particularly attractive since the optimal beam direction is typically nothing else but the direction of the receiver from the transmitter's point of view. One advantage with LoS beamforming is reciprocity (i.e., similarity of link characteristics) between the uplink and downlink directions in frequency division duplex (FDD) systems. Since the direct beam (direction) will comprise almost all of the signal energy, it is possible to use uplink beam estimates also for downlink transmissions only compensating for the phase difference arising from the frequency difference between the two links.
Other technologies, such as Global Positioning System (GPS), allow for highly accurate time displacement measurements for different receiver nodes relative to a transmitter node. Accurate time displacement is particularly necessary in long range communications where the link latency could otherwise destroy the connection due to, for example, latency differences exceeding a cyclic prefix length in Orthogonal Frequency Division Multiplexing (OFDM) based communications.
Combining what is known about the above-mentioned technologies, it is possible to derive the parameters for generating LoS beams for both the transmitter and receiver antenna arrays without the use of traditional pilot sequences. (Pilots are still necessary for phase information, though, although these could be spaced much more sparsely.) In a richly scattering environment, usually assumed in land based communications and well known in the art, a channel estimate, H, is estimated from pilot sequences received from various rays of the scattered transmitted signal. In such an environment, Multiple-Input Multiple-Output (MIMO) principles allow construction of an independent data stream (“layer”) from a linear combination of signals from all transmitter antenna ports by controlling the phase and amplitude of each antenna port's output so as to cause each of the received signals to contribute constructively at the UE.
By contrast, in a LoS environment with no surrounding scatterers, the only way to achieve a spatially diversified MIMO transmission is for the transmitted layers to be spatially separated at their source, in order for the receiver antenna array to be able to resolve the different transmitters. In order to do so, different layers are transmitted by different eNBs (or unique groups of eNBs) and resolved at the receiver by beamforming in their corresponding directions. With reference to FIG. 1, in order to form the receiver (or transmitter) beams, the temporal difference, Δt, between an arriving wave at two neighboring antenna elements may be expressed as,
      Δ    ⁢                  ⁢    t    =            d      c        ⁢    sin    ⁢                  ⁢    ϕ  where d is the antenna distance, c is the speed of light and φ is the angle of the impinging wave. Alternatively, this expression may be reformulated using a normalized antenna distance, k, d=kλ, and Δt=θλ/2πc, in which instead the phase difference, θ, between the two receiver elements becomesθ=2πk sin φ.
The corresponding beam forming is possible to perform at the transmitter side, both well known in the art.
The relation between the transmitted signal vector, x, and the received signal vector, y, in a MIMO system with the precoding matrix W, channel matrix H, and additive noise w isy=HWx+w. 
Furthermore, assuming fixed channel attenuation, the channel vector hi between the ith transmitter antenna and the N element receiver antenna array is
      h    i    =      [                            1                                      ⋮                                                  e                                          -                j                            ⁢                                                          ⁢              2              ⁢                                                          ⁢                              π                ⁡                                  (                                      N                    -                    1                                    )                                            ⁢                              θ                i                                                          ]  
Provided that all transmitter antennas are spatially separated, H will be orthonormal and hence invertible. Having obtained H, x may then be derived by, for example, zero forcing demodulation,x=(HHH)−1HH.
Hence, by using an antenna array within an eNB and a UE, correspondingly, it is possible to form both transmitter beams and receiver beams, and also to demodulate the transmitted signal by using location and velocity data of an aircraft.
Other techniques exist in order to estimate H, for example by use of pilot based channel estimation and channel state information (CSI) feedback. However, in this case, existing a priori codebooks that are based on the Rayleigh model may not suit the special LoS case that land-to-air communications comprise.
Today there exist two main systems for in-flight communications, that is, communications to and from the ground to a commercial airplane. One is ground based using a code division multiple access (CDMA) Evolution Data Optimized (EV-DO) link with capacity limited to 10 Mbps. This system comprises thousands of dedicated base stations covering all of the US. Being a third generation communications system, it does not live up to the data rates that are demanded from today's user, even less so for 300 passengers highly limited in their options to busy themselves at 30 000 feet. It is not feasible for a single eNB to achieve spatial diversity on its own due to the non-reflective surroundings of free-space. Even if the surroundings of the eNB were to allow reflections, resulting in multiple beams from one eNB reaching the aircraft, these would be so attenuated that their contributions would only be marginal in relation to the line of sight component.
The second group of systems is satellite based in which either the Ku band at around 15 GHz or the Ka band at around 30 GHz is used. These services will rely on geostationary, high throughput satellites (HTS) and will provide connections of up to 200 Mbps to airborne nodes. Although throughput may not be an immediate bottleneck for this design, the latency of such a system will in practice result in significantly lower speeds than what is expected for a corresponding land based system. HTS use a Multi-User MIMO (MU-MIMO) system of their own in spot beam forming whereby individual beams cover a specific geographical area of Earth, from approximately 75 000 km2 (roughly the size of South Carolina) and upwards, allowing the same time frequency resource to be reused multiple times in different beams. However, the satellite still only allows one beam to be directed at the same geographical area and the beam must be shared by all devices within that area, hence significantly reducing the capacity of a device. Cost, in terms of, for example, development, launching, manufacturing and maintenance is also a factor to be considered in a space based satellite system.
Hence there is a continued need for a standardized, affordable, ground based coordination method and/or device in which in-flight communications is managed by a ground-to-air-based system utilizing beams from multiple base stations on the ground in order to increase capacity in both downlink and uplink to airborne in-flight nodes.